The present invention relates to a magnetic field sensor, and more particularly to magnetic tunnel junction (MTJ) sensor that provides increased sensitivity with reduced hysteresis at a very small size.
MTJ devices are well known to show resistance changes as a function of applied magnetic fields. FIGS. 1A-1C are graphs showing typical output responses of MTJ devices with differing amounts of hysteresis (characterized by open loops in the resistance-versus-magnetic-field curves). FIG. 1A shows the output of a device with significant hysteresis which gives two different resistance values at the same magnetic field, depending on the past history of applied fields. The heavy dashed curve represents resistance values measured when the device is first saturated by applying a sufficiently large field in the negative direction—one that drives the device into the output region characterized by the upper closed, flat, line. Increasing the field monotonically now produces the resistance values of the dashed curve. Similarly the thin solid line represents resistances measured when the part is saturated in the positive direction and then the field is allowed to monotonically decrease. FIG. 1B shows the output of a device with less hysteresis than the output characteristic shown in FIG. 1A, but which has a less steep slope in the middle region of the curve, which results in less resolution of the sensor. FIG. 1C shows the output of a device that has no hysteresis at all, but which has an even less steep slope in the middle region of the curve.
A sensor with a response as shown in FIG. 1A is not well suited for a linear-sensor application whereas a sensor with a response as shown in FIG. 1C is much more ideal in this sense, as it gives a unique resistance value at each value of applied external field. However, the sensor response shown in FIG. 1C has low resolution—a sensor with high resolution would have a correspondingly high slope, or resistance change per change in applied magnetic field. The sensor response shown in FIG. 1B has a higher slope than that of FIG. 1C, but because it has some hysteresis, it can still be double valued over its sensitive region. These are the design trade-offs that have been involved in the design of MTJ sensors.
Hysteresis in a magnetic-field sensor further hinders good performance in that the sensitivity or resistance-versus-applied-field values (slope of the output curve) is also highly dependent on magnetic history. FIG. 2 is a graph of resistance versus applied field in a sensor having a hysteretic output response. As seen in FIG. 2, if the sensor is not driven to saturation then there are an infinite number of other secondary curves over which the resistance can trace. Each of these “minor loops” (loops traced when the magnetic thin film structure is not fully saturated, such as loop 20 in FIG. 2) fall within the envelope of the fully saturated or “major loop” characteristic. Therefore hysteretic devices will show minor-loop sensitivities that are reduced or, at best, no greater than the major loop sensitivity.
MTJ devices are typically comprised of a thin film stack of three functional layer groups—(1) a ferromagnetic “free layer” characterized by its ability to rotate freely in the plane of the thin film with the application of external fields to be measured by the sensor, (2) an insulating thin film that acts as a magnetic tunnel barrier, and (3) a ferromagnetic “pinned layer” whose magnetization vector is constrained by one of various techniques such that it does not rotate with the application of external fields to be measured by the sensor. The MTJ thin-film stack is photolithographically patterned into a suitable shape to create the sensor device. FIG. 3A is a top-down view, and FIG. 3B is a section view, of a typical sensor configuration, showing the magnetization directions of the layers. Contacts (not shown) are fabricated by any of various microelectronic techniques to make electrical connections to free layer 30 and pinned layer 32 of the stack. Current flows perpendicular to the plane of the thin films, across tunnel-barrier thin-film layer 34.
The configuration shown in FIGS. 3A and 3B has an output response as shown in FIG. 1A described above. With pinned layer 32 having a magnetization vector fixed in the same direction as the easy axis of free layer 30, and with no application of any effective orthogonal field (a field in the plane of the thin film but at 90 degrees from the easy axis of free layer 30), free layer 30 experiences switching along its easy axis.
FIG. 4 is a top-down view of an alternative sensor configuration, showing the magnetization directions of the layers, with an effective orthogonal field applied. The magnetization and field direction shown in FIG. 4 are identical to those shown in FIG. 3A, but an effective orthogonal field is applied in a direction as shown by arrow 40. As the effective orthogonal field is increased, hysteresis in the output response is suppressed (and correspondingly, the slope of the resistance output curve is decreased), as shown in FIGS. 1B and 1C described above.
In order to achieve a high resolution sensor, it is desirable to suppress magnetic hysteresis while achieving a relatively steeply sloped resistance output response.